Applicability

Use visitor pattern when:

• An object structure contains many classes of objects with differing interfaces, and you want to perform operations on these objects that depend on their concrete classes.
• many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid polluting their classes with these operations.
• the classes defining the object structure rarely change, but you often want to define new operations over the structure.

Structure

Participants

• Visitor: declares a Visit operation for each class of ConcreteElement in the object structure.
• ConcreteVisitor: implements each operation declared by Visitor.
• Element: defines an Accept operation that takes a visitor as an argument.
• ConcreteElement: implements an Accept operation that takes a visitor as an argument.
• ObjectStructure: can enumerate it's elements, may provide a high-level interface to allow the visitor to visit it's elements.

Collaborations

• A client that uses the Visitor pattern must create a ConcreteVisitor object and then traverse the object structure, visiting each element with the visitor.
When an element is visited, it calls the Visitor operation that corresponds to it's class.

Consequences

• Visitor makes adding new operations easy.
• A visitor gathers related operations and separates unrelated ones.
• Adding new ConcreteElement classes is hard.

Sample Code

Visitor

Related Patterns

• Composite: Visitors can be used to apply sn operation over an object structure defined by the Composite pattern.
• Interpreter: Visitor may be applied to do the interpretation.